if you're running at 500 FPS then yes a core2duo 2gb ram would be fine. if your budget can push it then go for a Intel Q6600, 2GB this will allow room for expanding and even allowing you to host other games if you ever wanted.
I suggest going for a 1 Gbit connection if that's possible with the host.
Also make sure to check out the network used, so you get good connectivity to the locations you need it for.
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Depends on your settings, but I believe for default server settings 100mbit will get you approx 200-300 slots. Only thing to keep in mind is custom maps etc. If you're hosting the maps on the server and 50 people have to download the map at the same time you may hit some bandwidth limitations. Honestly, I would go with 100mbit and just see how it goes.
Stick to 100mbit. There is no processor that can handle load for players that can waste more than 50mbit. Anyways, i'd recommend using linux because you will lose performance that you can use for at least 1 additional CSS server if you run windows.